Low Temperature CO Oxidation on Ruthenium Oxide Thin Films at Near-Atmospheric Pressures
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Low Temperature CO Oxidation on Ruthenium Oxide Thin Films at Near-Atmospheric Pressures Y. Martynova • B. Yang • X. Yu • J. A. Boscoboinik S. Shaikhutdinov • H.-J. Freund
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Received: 8 February 2012 / Accepted: 7 April 2012 / Published online: 24 April 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Ruthenium model catalysts in the form of thin ruthenium oxide films grown on Ru(0001) were studied in the CO oxidation reaction at near-atmospheric pressures. The surfaces were prepared under vacuum conditions prior to the reactivity measurements carried out in a circulating flow reactor using gas chromatography. The films possessing oxygen in amounts equivalent to 1–4 monolayers (MLE) on Ru(0001) as determined by electron spectroscopy, exposed both the oxidic (RuO2(110)-like) and O/Ru(0001) surfaces. In addition, one-dimensional oxide structures were observed by scanning tunneling microscopy, which are tentatively assigned to the intermediate state for a crystalline ruthenium oxide thin film that covered the entire surface at higher oxygen coverages. At low temperatures studied (400–470 K), the reaction sets in only in the presence of oxidic structures, i.e. when the oxygen coverage, on average, exceeds 1 MLE. The reaction rate slightly increases with increasing the nominal film thickness up to 7 MLE, reflecting primarily the lateral growth of oxide phases. The disordered oxide films showed even higher reactivity. The results suggest that surface ordering and oxide film thickness are not critical for the superior catalytic activity of ruthenium oxides in this reaction. Keywords CO oxidation Thin oxide films Ruthenium oxide
Y. Martynova B. Yang X. Yu J. A. Boscoboinik S. Shaikhutdinov (&) H.-J. Freund Abteilung Chemische Physik, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany e-mail: [email protected]
1 Introduction Well-ordered, thin oxide films have drawn some attention in recent years as suitable oxide supports for modeling highly dispersed metal catalysts at the atomic scale. It has been noticed, however, that the film thickness often influences the atomic structure and even the oxidation state of supported metal species due to the presence of a metal substrate underneath the film, which must be included in the proper description of such systems [1]. Not surprisingly, the ultra-thin oxide films exhibit physical and chemical properties different from the ‘‘thick’’ films which behave essentially as the bulk-like oxides [2, 3]. Indeed, our recent studies of iron oxide films grown on Pt(111) [4, 5] demonstrated that the film thickness may play an important role in oxidation reactions such as CO oxidation. At relatively low temperatures (*450 K), an ultra-thin FeO(111) film was found to be much more active than nmthick Fe3O4(111) films and a Pt(111) support under the same conditions [4]. It has turned out that a bilayer FeO film, which is inert at low oxygen pressures, transforms into a trilayer, O–Fe–O film at higher oxygen pressures and c
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